Unique Regenerative Mechanism to Replace Bone Lost During Dietary Bone Depletion in Weanling Mice

Conditional Deletion of Gremlin-1 in Cathepsin K-expressing Mature Osteoclasts Altered the Skeletal Response to Calcium Depletion in Sex-Dependent Manner

This study assessed the novel concept that osteoclast-derived Grem1 has regulatory functions in the skeletal response to calcium stress using an osteoclastic Grem1 conditional knockout (cKO) mouse model. The calcium stress was initiated by feeding cKO mutants and wildtype (WT) littermates a calcium-deficient diet for 2 weeks. Deletion of Grem1 in mature osteoclasts did not affect developmental bone growth nor basal bone turnover. In response to calcium depletion, male cKO mutants showed greater increases in osteoclastic resorption and trabecular bone loss than male WT littermates, indicating an enhanced skeletal sensitivity to calcium depletion in male mutants. The enhanced sensitivity to calcium depletion was sex-dependent, as female cKO mutants showed lower increases in osteoclastic resorption and bone loss than female WT littermates as well as male cKO mutants. The sex disparity in osteoclastic resorption response to calcium stress was intrinsic to osteoclasts since osteoclasts of male but not female cKO mutants showed greater in vitro bone resorption activity than osteoclasts of WT littermates of respective sex. Male cKO mutants displayed smaller bone formation response to calcium depletion than male WT littermates, while female mutants showed bigger bone formation response than female WT littermates, indicating that cKO mutants also displayed sex disparity in bone formation response. The sex disparity in bone formation response was not caused by intrinsic differences in osteoblasts but might be due to sex-dependent differential osteoclastic release of osteogenic factors. In summary, osteoclast-derived gremlin-1 has complicated and sex-dependent regulatory roles in skeletal response to calcium stress.

Defective bone repletion in aged Balb/cBy mice was caused by impaired osteoblastic differentiation

INTRODUCTION

This study was undertaken to gain mechanistic information about bone repair using the bone repletion model in aged Balb/cBy mice.

MATERIALS AND METHODS

one month-old (young) mice were fed a calcium-deficient diet for 2 weeks and 8 month-old (adult) and 21-25 month-old (aged) female mice for 4 weeks during depletion, which was followed by feeding a calcium-sufficient diet for 16 days during repletion. To determine if prolonged repletion would improve bone repair, an additional group of aged mice were repleted for 4 additional weeks. Control mice were fed calcium-sufficient diet throughout. In vivo bone repletion response was assessed by bone mineral density gain and histomorphometry. In vitro response was monitored by osteoblastic proliferation, differentiation, and senescence.

RESULTS

 There was no significant bone repletion in aged mice even with an extended repletion period, indicating an impaired bone repletion. This was not due to an increase in bone cell senescence or reduction in osteoblast proliferation, but to dysfunctional osteoblastic differentiation in aged bone cells. Osteoblasts of aged mice had elevated levels of cytosolic and ER calcium, which were associated with increased Cav1.2 and CaSR (extracellular calcium channels) expression but reduced expression of Orai1 and Stim1, key components of Stored Operated Ca2+ Entry (SOCE). Activation of Cav1.2 and CaSR leads to increased osteoblastic proliferation, but activation of SOCE is associated with osteoblastic differentiation.

CONCLUSION

The bone repletion mechanism in aged Balb/cBy mice is defective that is caused by an impaired osteoblast differentiation through reducedactivation of SOCE.

Calcium released by osteoclastic resorption stimulates autocrine/paracrine activities in local osteogenic cells to promote coupled bone formation

A major cause of osteoporosis is impaired coupled bone formation. Mechanistically, both osteoclast-derived and bone-derived growth factors have been previously implicated. We hypothesize that the release of bone calcium during osteoclastic bone resorption is essential for coupled bone formation. Osteoclastic resorption increases interstitial fluid calcium locally from the normal 1.8 mM up to 5 mM. MC3T3-E1 osteoprogenitors, cultured in a 3.6 mM calcium medium, demonstrated that calcium signaling stimulated osteogenic cell proliferation, differentiation, and migration. Calcium channel knockdown studies implicated calcium channels, Cav1.2, store-operated calcium entry (SOCE), and calcium-sensing receptor (CaSR) in regulating bone cell anabolic activities. MC3T3-E1 cultured in a 3.6 mM calcium medium expressed increased gene expression of Wnt signaling and growth factors platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and bone morphogenic protein-2 (BMP 2). Our coupling model of bone formation, the Receptor activator of nuclear factor-kappa-Β ligand (RANKL) treated mouse calvaria, confirmed the role of calcium signaling in coupled bone formation by exhibiting increased gene expression for osterix and osteocalcin. Critically, dual immunocytochemistry showed that RANKL treatment increased osterix positive cells and increased fluorescence intensity of Cav1.2 and CaSR protein expression per osterix positive cell. The data established that calcium released by osteoclasts contributed to the regulation of coupled bone formation. CRISPR/Cas-9 knockout of Cav1.2 in osteoprogenitors cultured in basal calcium medium caused a >80% decrease in the expression of downstream osteogenic genes, emphasizing the large magnitude of the effect of calcium signaling. Thus, calcium signaling is a major regulator of coupled bone formation.